Arsenic is a persistent, bio-accumulative toxin. At a pH of 8 and above, arsenic is readily soluble and thus transports easily through surface and ground water. The United States government drinking water standard for arsenic currently is 50 parts per billion (“ppb”), but is scheduled to be reduced to 10 ppb in 2006 due to its toxicity and possible links to cancer.
Current arsenic remediation technologies are relatively expensive, require substantial technical equipment and trained personnel to achieve significant reductions in arsenic levels, and are generally unsuitable for individual users, rural communities, or relatively smaller water systems. Lowering the federal water standard for arsenic will place significantly increased socio-economic pressures on those water systems that will be required to meet lower standard for arsenic.
According to estimates by the United States Environmental Protection Agency, a water standard of 5 ppb arsenic would cost consumers $374 million per year. In another estimate, the American Water Works Association has estimated a minimum cost of $1.4 billion per year, along with an initial capitalization cost of $14 billion, to meet a 5 ppb standard. An estimated 6,600 water systems nationwide serving at least 22.5 million people would be required to upgrade their existing systems to meet a 5 ppb standard.
When the scheduled 10 ppb federal drinking water standard for arsenic becomes effective, many water systems will be in immediate danger of being out of compliance. For example, the South Dakota Department of Environment and Natural Resources estimates that 30 (10%) of the state's public water systems would violate a drinking water standard for arsenic of 10 ppb. Although larger community water treatment plants in more populous states might be able to meet the proposed federal water standard with existing technology and personnel, smaller water systems and other water systems with limited financial and technical resources might not be capable of doing so. For example, such smaller and other water systems may include individual wells, rural communities, tribal water treatment facilities, urban communities with smaller populations (such as less than 10,000 people), urban communities lacking financial and technical resources to use existing technologies, and individuals desiring on-site arsenic removal regardless of water source.
The need for a low-cost, efficient arsenic removal system for such water systems is not unique to the United States. In many places throughout the world, excessive arsenic in potable water is a critical health issue, regardless of existing or non-existing regulations. The World Health Organization has compiled reports of relatively high levels of arsenic in drinking water in many countries, including Mexico, China, and Bangladesh.
Current remediation technologies commonly considered for removal or reduction of the amounts of arsenic in potable water include ion exchange, coagulation and filtration, activated alumina, lime softening, various iron based medium, and reverse osmosis. Each of these has significant shortcomings. For example, ion-exchange technology currently is used to remove or reduce the amounts of certain contaminants, including arsenic, in water. The removal of arsenic using this technology is based on the charge-charge interaction and thus it is not selective. Anionic ion-exchange resins remove not only arsenic but also other contaminants such as sulfate, selenium, fluoride, and nitrate. Also, suspended solids and iron precipitation can clog the system. In any event, an ion-exchange system must eventually be regenerated, typically by flushing with brine. This results in a concentrated brine solution containing high levels of arsenic and other contaminants, which in turn creates a waste disposal issue. Further an ion-exchange system does not provide an indication of the level of arsenic in the bed or of the bed being saturated with arsenic. Moreover, an ion-exchange system is too expensive, inefficient, and complex for use in smaller water systems or as an end-use application such as a home, farm, business, or individual well.
Coagulation and filtration is a batch process involving segregating a fixed amount of arsenic-contaminated water into a tank, adding iron to coagulate the arsenic, and filtering the batch to remove the coagulated arsenic. This process requires significant capital equipment and trained personnel, and is most efficient at a mid-range pH. As a non-continuous process that is relatively expensive and complex, coagulation and filtration also is unsuitable for smaller water systems or as an end use application.
Both ion-exchange technology and coagulation and filtration have been shown to reduce arsenic in water to about 2 ppb. However, both techniques are more effective when arsenic is in the form of As(V). If As(III) is present, it must first be oxidized to As(V), which adds a pretreatment step, greater costs, and greater technical resources of equipment and personnel. In addition, requirements and expense of the disposal of the resulting arsenic-contaminated sludge must be considered.
Lime softening is a process in which highly trained personnel adjust the pH of the arsenic-contaminated water to a relatively high pH, which facilitates the adsorption of arsenic onto larger particles, such as iron hydroxide, and then reduces the remaining water to a potable pH level. As with the ion-exchange and the coagulation and filtration technologies, lime softening creates a waste product that results in disposal issues, is relatively expensive, requires trained personnel to operate the equipment, and is not a continuous process.
Activated alumina, reverse osmosis, and a variety of other technologies utilizing iron-based medium are other processes that are currently considered for removal or reduction of arsenic in drinking water. Activated alumina requires significant technical intervention and processing, making it impractical for all but larger water systems. Reverse osmosis is not an effective process for this purpose because up to 80 to 90% of the water is discarded. Iron-based media generally involve the use or iron oxide, e.g., sand coated with rust, to attract, remove, and hold arsenic from the water. These processes generally have significant problems with capacity, water, quality, efficiency, and waste disposal. Although having a high capacity for arsenic, granulated ferric hydroxides (“GFH”) are extremely expensive and must be disposed of in a certified landfill or recycled industrially. Additionally, granulated ferric hydroxides require substantial technical oversight and are unsuitable for rural and small public water supply systems.
In an industrial application, a published Japanese Patent Application (No. 1-127094) disclosed the use of porous coral limestone in a process to remove arsenic from a waste liquid by absorption of arsenic. In general, the Japanese publication disclosed the use of porous limestone having a void ratio of 10 to 50 cm3/g and particle diameters of approximately 0.1 to 4.5 millimeters. However, this application appears to have used the term, “void ratio”, to refer to the reciprocal of the material's density. The term “void ratio”, also known as “porosity”, used in the art is defined as a ratio of the volume of voids verse the total volume of a rock, is usually expressed as a decimal fraction or a percentage, and is dimensionless. Assuming that it is the case, then the coral limestone used in the application has a very low density, preferably from 0.02 to 0.1 g/cm3. Because additives to the porous limestone absorb arsenic on the surface and in the pores, the amount of arsenic absorbed varies depending on the void ratio of the limestone. When porous limestone is crushed, the pore size eventually decreases, thereby reducing its ability and capacity to absorb arsenic. To improve arsenic absorption, it disclosed that additional process treatments or the addition of specific chemical agents were required, namely ferric chloride, aluminum sulfate, magnesium nitrate, ferric hydroxide, or glutaraldehyde.
Therefore a need exists for a method and composition to reduce the amounts of arsenic in arsenic-contaminated water, particularly with less expense, less complexity, less personnel requirements, and less waste disposal issues. With arsenic levels in drinking water increasingly becoming a health concern in the United States and elsewhere, and with a possible significant reduction in the federal water standard for arsenic in drinking water, this need is particularly acute for home, individual, rural, and relatively smaller drinking water systems.